Data processing systems with format-preserving encryption and decryption engines
View Patent ↗A data processing system is provided that includes format-preserving encryption and decryption engines. A string that contains characters has a specified format. The format defines a legal set of character values for each character position in the string. During encryption operations with the encryption engine, a string is processed to remove extraneous characters and to encode the string using an index. The processed string is encrypted using a format-preserving block cipher. The output of the block cipher is post-processed to produce an encrypted string having the same specified format as the original unencrypted string. During decryption operations, the decryption engine uses the format-preserving block cipher in reverse to transform the encrypted string into a decrypted string having the same format.
1. A method for encrypting a data string to produce a corresponding encrypted data string using a format-preserving block cipher on a data processing system, comprising:
wherein the data string contains characters and has a format specifying a legal set of character values for each of the characters of the data string, wherein the data string contains a first string of characters that has a first format specifying a legal set of character values for each of the characters of the first string and contains a second string of characters that has a second format specifying a legal set of character values of each of the characters of the second string;
with a format-preserving combining operation, combining a first subkey with the first string of characters while preserving the first format of the first string of characters; and
with the format-preserving combining operation, combining a second subkey with the second string of characters while preserving the second format of the second string of characters.
2. The method defined in claim 1 , further comprising:
before combining the first subkey with the first string of characters, encoding the data string using at least one index of sequential index values each of which corresponds to a respective one of the character values in the legal set of character values for the characters of the data string.
3. The method defined in claim 2 , further comprising:
using the index, decoding the encrypted data string to produce a decoded encrypted data string with characters having character values in the legal set of character values.
4. The method defined in claim 3 further comprising:
generating the first and second subkeys using a subkey generation algorithm; and
receiving a key as an input with the subkey generation algorithm.
5. The method defined in claim 4 wherein generating the first and second subkeys using the subkey generation algorithm comprises using a cryptographic hash function to generate the first and second subkeys.
6. The method defined in claim 4 wherein generating the first and second subkeys using the subkey generation algorithm comprises using a cryptographic message authentication code function to generate the first and second subkeys.
7. The method defined in claim 1 further comprising decrypting the encrypted string using a subkey generation algorithm that generates the first and second subkeys and using addition mod x, where x is an integer.
8. The method defined in claim 1 wherein at least one of the legal sets of character values of the data string comprises letters.
9. The method defined in claim 1 wherein, for at least one of the characters in the data string, the legal set of character values for that character comprises ten character values.
10. The method defined in claim 1 wherein the format-preserving combining operation comprises addition mod x, where x is an integer.
11. The method defined in claim 1 , wherein combining the first subkey with the first string of characters while preserving the first format of the first string of characters produces a corresponding third string of encrypted characters in the first format, the method further comprising:
using the format-preserving combining operation to combine a third subkey with the third string of encrypted characters while preserving the first format of the third string of encrypted characters.
12. The method defined in claim 11 , wherein combining the second subkey with the second string of characters while preserving the second format of the second string of characters produces a corresponding fourth string of characters in the second format, the method further comprising:
using the format-preserving combining operation to combine a fourth subkey with the fourth string of characters while preserving the second format of the fourth string of characters.